When Flow (or any extension) is undeployed, Echo events/notifications that it generated remain behind in the database. This is harmless for most extensions (Echo hides them), but in Flow's case, they include serialized data (instances of the Flow\Model\UUID class), which can no longer be unserialized once Flow is gone, which is causing warnings to be logged (T388725#10663772), such as:
Unserializing of extra data partially failed for event <id> of type flow-post-reply
Unserializing of extra data partially failed for event <id> of type flow-new-topic
Unserializing of extra data partially failed for event {id} of type {type}We should have a maintenance script that removes these events, and run it once Flow is undeployed.